The invention relates to converting one page description representation of a document into another page description representation. More specifically, a method and device to convert a page sequential representation into a page independent representation is described.
Digitally stored documents are often represented in an output-device independent language known as a page description language (PDL). Well known PDLs include the POSTSCRIPT language and the PORTABLE DOCUMENT FORMAT (PDF) language. Both POSTSCRIPT and PORTABLE DOCUMENT FORMAT are trademarks of Adobe Systems Incorporated, San Jose, Calif. In a PDL representation, a document may be described as lists of instructions and resources. Instructions, for example, include commands to draw a line between two points or to fill a region with a specified color. Resources, for example, include images (raster representations of displayable objects), color spaces, and font specifications.
Page sequential PDL representations are characterized by the fact that a resource used on page N may have been specified/defined on a prior page. In these situations, only a reference to the earlier specification is made on page N. This characteristic makes the document representation relatively compact because resources are not duplicated at every point in the file they are used. It also means that, in general, page N cannot be output (e.g., printed or displayed on a display screen) without first processing all (N-1) prior pages. POSTSCRIPT is a page sequential representation.
In a page independent PDL representation, every page that uses a resource has sufficient information to locate the resource in the file without processing preceding pages of the file. This allows any page in a document represented in a page independent PDL to be printed independently of any other page in the document. PDF is a page independent representation. It is the page independent characteristic of the PDF representation that is exploited in high-speed print environments.